Validation rule that excludes comma but allows fullstops

Validation rule that excludes comma but allows fullstops

I have a field that needs to allow just numbers and fullstops (as could be long decimal) and have tried using the validation rule 

contains - ,,/,£,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z

I chose these as they are the most common things entered. However, it does not recognise the comma. It works for everything else. How do I add comma as an excluded character?